The manner of prior incorporation of photographically useful agents in a photographic material and thereby to fully achieve their effects differs from the manner in which they are used in processing solutions. For instance, photographic agents of the kind which cannot withstand long-range storage in processing solutions because of their liability to decomposition under acid-alkali or oxidation-reduction conditions can be utilized effectively and, at the same time, the composition of a processing solution can be simplified to facilitate the preparation thereof. Further, in processing a light-sensitive material, it becomes feasible for a desired photographic agent to fulfill its function at a desired time or/and at a desired place, that is to say, only in a specific layer or layers in the vicinity thereof where the light-sensitive material has a multilayer structure, or for a large amount of a desired photographic agent to change as a function of silver halide development. However, if the photographic agent is added to a photographic material in an active form, it undergoes reactions with other components present in the photographic material or decomposes under the influence of heat or oxygen during storage prior to processing. Therefore, it becomes impossible to achieve fully the expected capabilities at the time of processing. One solution to this problem is a method in which a photographic agent is converted into a substantially inactive form by blocking the active group, that is, a precursor thereof, and then, the precursor is added to a photographic material. When a useful photographic agent is a dye, a functional group having a great effect on spectral absorption of the dye is blocked and thereby its spectral absorption is shifted to the shorter or the longer wavelength side. Under this circumstance, even if the blocked dye is also present in a silver halide emulsion layer with a spectral sensitivity in the wavelength region corresponding to the spectral absorption of the original dye, a lowering of sensitivity due to the so-called filter effect does not occur. Therefore, it can be used advantageously. When a photographically useful agent is an antifoggant or a development inhibitor, blocking of the active group can offer many advantages, e.g., desensitization due to adsorption onto light-sensitive silver halide grains and formation of silver salts upon storage can be inhibited, and at the same time, through timely release of such photographic agents, fog can be reduced without impairing photographic speed, fog arising from overdevelopment can be depressed, development can be stopped at a desired time, and so on. When a photographically useful agent is a developer, assistant developer or a fogging agent, blocking the active group or the adsorptive group can offer the advantages that various photographically adverse effects due to conversion into semiquinones or oxidants through air oxidation on storage can be prevented, or injection of electrons into silver halides can be prevented from occurring during storage. Thereby, generation of fog nuclei can be inhibited. This results in the realization of stable processing, and the like. Where a photographically useful agent is a bleach accelerator or a bleachfix accelerator, blocking the active group can offer the advantages that in storing the sensitive material, reactions with other components also present with such an agent can be suppressed, while in processing it, the expected ability can be brought into full play upon removal of the blocking group at the time needed.
As described above, to use a photographic agent in the form of a precursor thereof turns out to be an extremely effective means of freely achieving the ability of the photographic agent. However, precursors thereof have very severe requirements. That is, they must satisfy two contradictory requirements--one requirement being it is stable during storage, and the other requirement being rapid and highly efficient release of the photographic agent by removal of the blocking group at a desired time upon processing.
Several techniques for blocking photographic agents are already known. For example, well-known techniques involve utilization of a blocking group such as an acyl group, a sulfonyl group or the like, as described in Japanese Patent Publication No. 44805/72; utilization of a blocking group which releases a photographic agent due to the so-called reverse Micheal's reaction, as described in Japanese Patent Publication Nos. 39727/79, 9696/80 and 34927/80; utilization of a blocking group which releases a photographic agent by an intramolecular electron transfer accompanying the production of quinonemethide or the analogues thereof, as described in Japanese Patent Publication No. 39727/79 and Japanese Patent Application (OPI) Nos. 135944/82, 135945/82 and 136640/82 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application"); utilization of the intramolecular ring closure reaction described in Japanese Patent Application (OPI) No. 53330/80; utilization of the cleavage of a 5- or 6-membered ring described in Japanese Patent Application (OPI) Nos. 76541/82, 135949/82 and 179842/82; and so on. These known techniques have the disadvantage that those which are stable under storage conditions release photographic agents too slowly at the time of processing and, therefore, they require the processing under high alkalinities of a pH of 12 or above, while those which have sufficiently high release speeds at the time of processing under alkaline conditions of a pH of 9 to 12 decompose slowly under storage conditions thereby impairing their function as a precursor. This disadvantage is believed attributable to the dependence of the release of a photographically useful agent from a blocked photographic agent thereof due to attack of OH.sup.- ion. More specifically, the development processing of generally used silver halide photographic materials is carried out using a developing solution having a pH of 9 to 12, and under these circumstances, a difference in OH.sup.- ion concentration between the time of storage of the photographic material (pH: about 6 to 7) and the time of processing (pH: 9 to 12) ranges from 10.sup.2 to 10.sup.5. Accordingly, a blocked photographic agent capable of releasing a photographically useful agent in a half-life period of, e.g., 3 minutes (which means that 3 minutes is required for the amount of the blocked photographic agent to be reduced by decomposition to one-half the amount present thereof) upon processing under the condition of, e.g., pH=10, can be estimated to decompose upon storage under the condition of pH=6 in a half-life period of 3 min.times.10.sup.4 =30,000 min or 500 hr. This means that half the amount of the blocked photographic agent present decomposes during storage over a period of about three weeks. Consequently, such a blocked agent cannot possibly be suitable for practical use. In addition, although the half-life period of the decomposition upon storage becomes about 30 weeks (10 times the above-described amount) when a blocked photographic agent having a half-life period of 3 minutes with respect to release upon the processing under the condition of pH=11 is employed, even this numerical value of 30 weeks is quite unsatisfactory. This also is difficult for practical use from the viewpoint of maintaining stability.